Donald Trump: I've been 'a little childish'

New York (CNN)Donald Trump admitted Monday that his rhetoric on the campaign trail may not always sound entirely presidential.

"Probably is a little childish," Trump said in an interview with CNN's Erin Burnett at Trump Tower in New York. "But you know what? This is a campaign."
The billionaire businessman defended his style, saying the attacks he's unleashed on his rivals in the Republican presidential field have largely been reactive.
"I'm responding to them. I'm a counter-puncher," Trump said. "For instance, (Gov. Scott) Walker was very nice to me. All of a sudden, he hit me and I hit him back. All of these guys. (Sen. Marco) Rubio was very nice to me, couldn't have been nicer. All of a sudden a week ago he started hitting me."
    Trump has taken shots at many of his competitors -- particularly going after former Florida Gov. Jeb Bush, Rubio and Kentucky Sen. Rand Paul in recent weeks.
    He also frequently refers to Democratic front-runner Hillary Clinton as the "worst" secretary of state in the country's history -- a charge that he repeated on Monday.
    "We look at the job that Hillary did as secretary of state -- she goes down as perhaps the worst secretary of state in history and when I run against her evenly in the polls, I'm doing very well against Hillary and beating her," Trump said.
    (A Quinnipiac University poll last week had Trump trailing Clinton in a head-to-head matchup, 43% to 45%).
    Monday's interview comes as Trump continues to lead in the national polls, though his numbers have begun to slip in some surveys following CNN's Republican debate in California earlier this month.
    Pressed by critics and pundits to show more substance, Trump on Monday released a tax plan that proposed slashing income taxes for Americans across the board.
    The looming question he must now answer is how he would make up for the lost revenue from these deep tax cuts. The real estate magnate says his plan would boost the overall economy by bringing jobs back from overseas and stimulating investments and growth at home.
    "The economy's going to just be absolutely like a rocket," he said. "This is what I'm good at, this is really my wheelhouse."
    One measure in Trump's tax proposal aimed at bringing in revenue is the abolishment of the so-called "carried interest" loophole. It's a popular idea among the 2016 White House contenders that would tax the profits for some investors like hedge fund managers at a higher rate.
    Trump acknowledged Monday that closing the carried interest loophole isn't enough -- but that it's an important thing to do "psychologically."
    "When you have a hedge fund guy who's making $200 million a year and ... he's paying a very low rate of taxes, it's not fair and I think it says a lot," he said. "I think it tells people a lot and it's got to end."
    Trump also weighed in on the disagreement between President Barack Obama and Russian President Vladimir Putin on the best approach to take with Syrian President Bashar al-Assad, as the United States confronts the threat of ISIS.
    His solution? Let Russia and ISIS have at it.
    "Let Russia fight ISIS if they want to fight them," he said. "Russia likes Assad seemingly a lot. Let them worry about ISIS. Let them fight it out."